Wrong order of commands execution in TimerTask.run() (in Android app written in Java)

I’m writing a simple android app (in Java) that should be able to load a *.txt file and display its content one word at a time every 0.5 second. I’ve managed to put content of a text file into an String array which under each index stores one word. I’m using Timer and TimerTask to display words from the array in a TextView with given interval. To achieve this goal my TimerTask run() method first calls TextViev.setText() method that displays item from the array at current index and then increments index (or so I thought). As it turns out first the index is incremented and then TextViev.setText() method is called (despite the order of commands in overridden run() method of TimerTask). My question is: why commands are executed in this order, and how can I fix it?

TimerTask readingTimerTask = new TimerTask() {
            @Override
            public void run() {
                if (wordIndex < textArr.length - 1) {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            tvWords.setText(textArr[wordIndex]);
                            Log.i(LOG_TAG, "setText() called. wordIndex: " + String.valueOf(wordIndex));
                        }
                    });
                    wordIndex++;
                    Log.i(LOG_TAG, "wordIndex incremented. New value of wordIndex: " + String.valueOf(wordIndex));
                } else {
                    stopReading();
                }
            }
        };
mTimer = new Timer();
Log.i(LOG_TAG, "Timer is starting. wordIndex: " + String.valueOf(wordIndex));
mTimer.schedule(readingTimerTask, 100, displayTimeInMs);

And the Logcat looks something like this:

D/MainActivity: wordIndex in onActivityResult (file loaded): 0
I/MainActivity: Timer is starting. wordIndex: 0
I/MainActivity: wordIndex incremented. New value of wordIndex: 1
I/MainActivity: setText() called. wordIndex: 1
I/MainActivity: wordIndex incremented. New value of wordIndex: 2
I/MainActivity: setText() called. wordIndex: 2
I/MainActivity: wordIndex incremented. New value of wordIndex: 3
I/MainActivity: setText() called. wordIndex: 3
I/MainActivity: wordIndex incremented. New value of wordIndex: 4
I/MainActivity: setText() called. wordIndex: 4
I/MainActivity: wordIndex incremented. New value of wordIndex: 5
I/MainActivity: setText() called. wordIndex: 5
I/MainActivity: Timer has been stopped. wordIndex: 5


</details>


# 答案1
**得分**: 0

**为什么要按这个顺序执行命令？**

当从后台线程调用`runOnUiThread()`时，它不会等待`run()`内的命令完成后才返回。它会安排这些命令在一个单独的线程上执行：UI 线程。

我假设你的代码片段在 UI（或“主”）线程上被调用。与此同时，你的`TimerTask`引入了第二个线程：它的内部后台工作者线程。换句话说，你创建了一个[竞态条件](https://en.wikipedia.org/wiki/Race_condition#Example)。

`wordIndex++`在这种情况下不是_保证_会首先执行，但通常会在竞争中获胜，因为 UI 线程需要一点时间来处理`Runnable`。

**如何修复这个问题？**

你可能并不打算在程序中引入多个线程 —— 那会使事情变得非常复杂，这并不适合新手程序员。你可能希望使用[`CountDownTimer`](https://developer.android.com/reference/android/os/CountDownTimer)，而不是`TimerTask`。这将帮助你保持一切在 UI 线程上执行。

<details>
<summary>英文:</summary>

**Why are the commands executed in this order?**

When called from a background thread, `runOnUiThread()` does not wait for the commands within `run()` to complete before returning. It arranges for them to execute on a _separate thread_: The UI thread.

I assume that your code snippet is invoked on the UI (or &quot;main&quot;) thread. Meanwhile, your `TimerTask` is introducing a second thread: Its internal background worker. IOW, you have created a [race condition](https://en.wikipedia.org/wiki/Race_condition#Example).

The `wordIndex++` is not _guaranteed_ to execute first, under such a condition, but it will usually &quot;win&quot; the race, since it takes a little while for the UI thread to handle the `Runnable`.

**How can I fix it?**

You might not&#39;ve intended to introduce multiple threads into your program -- that can make things quite complicated, and it is not for novice programmers. You may want to use a [`CountDownTimer`](https://developer.android.com/reference/android/os/CountDownTimer), instead of a `TimerTask`. That will help you keep everything on the UI thread.

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